A wireless local-area network (WLAN) links wireless-enabled devices using a distribution method (such as orthogonal frequency-division multiplexing (OFDM). A WLAN allows users to move around an area serviced by the WLAN while still maintaining interconnectivity. This is one reason why WLANs have become increasingly popular with consumers both in the home and in commercial and public areas.
Multiple-input and multiple-output (MIMO) technology MIMO is an emerging technology that can significantly boost network capacity by exploiting the spatial properties of wireless channels on the WLAN. It has been included in several wireless standards, most notably the latest ratified Institute of Electrical and Electronics Engineers (IEEE) IEEE 802.11n standard. Many commercial wireless devices are already using this standard. If a device using this standard has two antennas when communicating with an access point having multiple antennas the device can transmit separate frames on each antenna simultaneously. This can potentially improve its link capacity by a factor of two.
Information theory indicates that it is possible for multiple stations to form a “virtual MIMO” system in which the stations transmit simultaneously. This is called spatial multiple access. In this type of network the access point may still be able to decode all frames correctly as long as the number of concurrent frames is less than the number of antennas at the access point. With spatial multi-access all stations can transmit simultaneously to make full use of the access point's antennas. Thus, the network capacity can increase linearly with the number of antennas at the access point. The access point is precisely the device in the network that can best accommodate the cost, size, and power of a relatively large number of radios.
Conventional single user MIMO systems support only point-to-point communication, where the sampling clocks for all antennas at the transmitter (or receiver) are completely synchronized. With single-user MIMO, the capacity improvement is bounded by the number of transmitter or receiver antennas (whichever is smaller). In practice, due to size, cost, and power limitations, mobile stations generally have only a few antennas. For example, most IEEE 802.11n devices have only two antennas. However, access points do not have the same constraints as mobile stations and can be generously provisioned with resources. This means that an access point can potentially have a much larger number of MIMO radios. Nevertheless, the network capacity will not improve beyond the link capacity as it is constrained by the number of antennas at the mobile station. Therefore, in practice, the number of antennas at the mobile station usually constrains the network capacity.
In contrast, multi-user MIMO (MU-MIMO) systems allow multiple stations to transmit concurrently thereby fully utilizing the antennas of the access point. However, most MU-MIMO systems are based on cellular networks, where the central base station strictly controls and tightly synchronizes all mobile stations. This way the central base station can precisely measure the spatial signatures, including the wireless channels and the delay of the radio signal from sending antenna to each receiving antenna. With such information data streams from different users can be separated using a linear decorrelator or other algorithms.
Thus, spatial multiple access holds the promise to boost the capacity of WLANs when an access point on the WLAN contains multiple antennas. Due to the asynchronous and uncontrolled nature of WLANs, however, conventional multiple-input and multiple-output (MIMO) technology does not work efficiently when concurrent transmissions from multiple stations on the WLAN are uncoordinated or uncontrolled.